1. Field of the Invention
The invention relates generally to a vehicle, in particular an industrial vehicle and more particularly to a vehicle comprising a system for reducing roll and/or pitch movement of the vehicle when the vehicle is traveling over a surface having surface unevenness.
The invention also relates to a method for reducing roll and/or pitch movement of a vehicle traveling on a surface having surface unevenness.
2. Description of the Related Art
As travel surfaces are never perfectly smooth, a traveling vehicle unavoidably encounters unevenness, which causes back-and-forth movement of the vehicle body (“pitch”) and a sideways rocking motion of the vehicle body (“roll” or “body roll”). These motions are a cause of discomfort to passengers traveling in the vehicle, as well as a safety concern, as these motions tend to reduce the grip of the wheels onto the traveling surface.
Even very early incarnations of the automobile had some provision for reducing the body motions of the traveling vehicle. For some time leaf springs, adopted from horse drawn carriages, were used for this purpose.
Since decades, motor vehicles, such as cars, vans and trucks, have been provided with a combination of springs and shock absorbers (“dampers”). The springs soften the transmission of wheel movements to the vehicle body; the shock absorbers dampen the sinusoidal motion of the springs. Mechanically the use of springs splits the vehicle in an “unsprung” portion (basically, the wheels) and a “sprung” portion (basically the rest of the vehicle). It has long been recognized that the body motions of the vehicle can be reduced by reducing the weight of the unsprung portion of the vehicle relative to the sprung portion. This insight has led to the use of pneumatic tires and light weight alloy wheel rims as weight-reducing measures. In addition, the use of rubber tires, in particular pneumatic tires, imparts a certain measure of “springiness” to the wheels, so that the wheels are not fully “unsprung.”
The characteristics of the springs and the shock absorbers represent a compromise between passenger comfort on the one hand, and handling of the vehicle on the other. Depending on the target demographics a car manufacturer may tune the suspension of the vehicle to favor comfort or handling. Certain upscale models permit the operator of the vehicle to choose between two or more pre-set suspension tunings. For example, the operator of the vehicle may select a stiff suspension tuning for sport driving, and a more moderate tuning for long distance driving and/or poor road conditions.
Systems have been proposed for adjusting the tuning of the suspension of a road vehicle in response to a detected unevenness in the road surface. U.S. Pat. No. 5,347,457 to Tanaka et al. discloses a vehicle equipped with a sensor at the front of the vehicle. The sensor registers an unevenness, such as a protuberance, in the road surface. The vehicle has springs and dampers of which the rigidity can be adjusted. A controller changes the rigidity of the spring and the damper of a specific wheel at the time the particular wheel reaches the protuberance in the road surface.
Likewise, JP 6-024233 to Nissan Motor discloses a vehicle with a sensing means for sensing a road surface ahead of the wheels. When an unevenness is detected, a controller excites the sub-sprung motion part into a vibration in a phase matching that of the unevenness detected in the road surface.
WO 2005/014315 A1 to Continental Aktiengesellschaft discloses shock absorbers that are adjusted to pavement conditions. The pavement conditions are registered by a sensor placed at the front end of the vehicle.
In an alternate approach the position of a wheel relative to the vehicle body is changed in anticipation of the wheel's encountering a bump or a hole in the road surface. As a result the wheel's vertical motion, necessary for dealing with the hole or bump, is not shared by the vehicle body.
This is the approach underlying the system disclosed in DE 101 57 426 A1 to Brötz. This reference discloses a vehicle in which each wheel has a dedicated sensor for determining bumps and recesses in the road surface in front of the wheel. The vehicle has means for individually raising each wheel when it encounters a bump, and for lowering each wheel when it encounters a recess.
EP 1 449 688 A2 to Bose Corporation discloses an active suspension system for a vehicle, including elements for developing and executing a trajectory plan responsive to the path on which the vehicle is traveling. Road surface information may be stored in memory on board the vehicle, or may be obtained wirelessly from an external database. The location of the vehicle is determined by use of a global positioning system, by dead reckoning, or a combination of the two.
A different approach is disclosed in DE 39 30 517 A1 to Robert Bosch GmbH. This reference discloses a vehicle with three sensors associated with each wheel. The sensors monitor, respectively, the damper force, the distance between the wheel and the vehicle body, and the (vertical) acceleration of the wheel. These three inputs are used to calculate a road surface unevenness velocity, which is input to the controller as a disturbance quantity. The calculation requires two time integrations and the estimation of three system parameters: suspension stiffness, tire stiffness and wheel mass.
Industrial vehicles, such as fork lift trucks, present a unique set of challenges with respect to controlling pitch and roll motions. Such vehicles generally travel on concrete floors that are, compared to highways, relatively smooth. In addition, such vehicles travel at relatively low speed, for example up to 12 km/hr.
Nevertheless, even such concrete floors inevitably present surface unevenness, often in the form of undulations. Fork lift trucks carry heavy loads, often at great heights, for example up to 17 meters. In addition, fork lift trucks such as “Narrow Aisle” trucks and “Very Narrow Aisle” trucks operate in tight spaces between tall storage racks, leaving little room for lateral movement. There is a compelling economic need to operate these trucks at as high a speed as possible, to reduce the time for loading and unloading to the minimum necessary or to maximize the number of pallet picks per unit of time.
These factors combine to a compelling need for providing a system for reducing roll and pitch motion of a moving vehicle that is particularly suitable for industrial vehicles, such as fork lift trucks.
Methods for avoiding roll and/or pitch movement of an industrial vehicle when the vehicle is traveling over an undulating floor are known to the person skilled in the art. U.S. Pat. No. 3,937,339 for example describes such a method wherein the industrial vehicle is an outdoor lifting vehicle and comprises a chassis with four supporting wheels for supporting the vehicle on the floor at a supporting location along a supporting path of the outdoor undulating floor. The vehicle comprises a sensor for sensing undulations in the floor in the supporting path of the supporting wheel while traveling over the undulating floor, and roll and/or pitch compensation means for compensating the roll and/or pitch movement of the vehicle caused by the undulations in the floor. The sensor is in the form of an electric circuit with level sensing means responsive to the transverse orientation of the vehicle. The roll and/or pitch compensation means for compensating the roll and/or pitch movement of the vehicle caused by the undulations in the floor is in the form of hydraulics controlling the position of the vehicle supporting wheels with respect to the chassis of the vehicle. It is the aim of the method to keep the vehicle, more in particular the chassis of the vehicle, as level as possible.
However, the electric circuit is only provided for sensing the transverse orientation of the vehicle at the supporting locations of the supporting wheels of the vehicle. Therefore, future roll and/or pitch movements of the vehicle cannot be taken into account and therefore a sufficient avoidance of roll and/or pitch movements of the vehicle can only be obtained when the vehicle is either driving relatively slow or when it is stationary. It is however often required that industrial vehicles drive at higher speeds. Moreover, it has been found that rolling movement causes, especially at higher speeds of the vehicle, the vehicle load to move transversally and can even cause the load lifted by the vehicle to move according to an eigenfrequency, which can even be dangerous as it may cause the load, or even the vehicle, to tip over, which is much undesired.
Thus, there is a particular need for a system for reducing roll and pitch in a moving vehicle responsive to actual and predicted pitch and roll motions of the vehicle.
There is a further need for such a system requiring a minimum number of sensors and a minimum amount of computing power.
There is still a further need for such a system suitable for vehicles having a rigid connection between the wheels and the chassis.